Modern motor vehicles are equipped with comfort systems, for example power windows or sunroofs. These permit the windows and sunroof to be automatically opened and closed. The manual operation is controlled for example with the aid of rocker switches with three switch positions. One switch position activates the opening action, one activates the closing action, and another switches off the drive motor that generates the respective movement.
DE-A 198 09 628 relates to a drive unit for moving a component, for example a window, a seat, or a sunroof of a motor vehicle. The drive unit has an electric motor with for example three field coil pairs, which is brushless and can therefore be supplied by an electronic commutation circuit. A safety circuit switches the electric motor off for example if the moving component begins to pinch a human appendage. The safety circuit reacts to the frequency of signals obtained from current or voltage fluctuations in at least one electrical transmission line of the motor.
The subject of DE-A 199 01 855 is a method and device for operating an actuating drive unit in a motor vehicle. The method is comprised among other things of the fact that an actuating force exerted by the actuating drive unit is measured, this actuating force is compared to a limit force, and the actuating drive unit is reversed if the limit force is exceeded (closing force limitation).
DE-A 30 34 118 relates to a method for electronically actuating and monitoring the opening and closing cycle of electrically actuatable units, for example power windows and electric sunroofs, in particular of motor vehicles, as well as an electrical circuit arrangement for executing the method. An electrical actuating drive unit connected to the dc-powered electrical system can be actuated by means of electrical switches, in particular push buttons. In order to avoid safety defects (the pinching of at-risk appendages such as the neck, head, fingers, etc.), the path traveled is electronically detected during the opening of the unit, this detected opening path is compared to the closing path traveled during the closing of the unit, and the speed of the electric actuating drive unit during closing is detected and electronically compared to a constant standard value. When there is a reduction in speed during closing, the actuating drive unit is switched off. This is intended to reduce the safety defects such as the pinching of at-risk appendages during the closing of the electrically actuatable units.
In the prior art, the actuating drive unit is also switched off if one of the electrically actuatable units jams, in order to thus prevent a mechanical or electrical overload of the actuating drive unit. This switching off is triggered after a predetermined jamming switch-off time has elapsed in which the unit has been jammed in the opening or closing direction. Usually the jamming switch-off time is predetermined as a period of between 100 ms and 500 ms.
This jamming switch-off time, however, can be disadvantageous if a reversing of the actuating drive unit is triggered (for example due to a pinching emergency), but is not possible because of temporary voltage dips in the operating voltage. If the voltage dips last longer than the jamming switch-off time, then the evaluation electronics switch the drive unit off in the same way as when there is a jam. A pinched appendage is therefore no longer released.
Then a reversal from the closing action to the opening action of the actuating drive unit is not possible, for example, if the operating voltage (e.g. the electrical system voltage of a motor vehicle) temporarily drops below the relay pick-up voltage of the relay that is used to switch to the opening action. In this connection, the relay pick-up voltage is understood to be the value of the lowest voltage that causes a relay to switch. Among other things, this relay pick-up voltage depends on the additional wiring (for example a protective diode before the relay winding), the ambient temperature, the self-heating, and the internal resistance of the relay driver, and is typically between 8 and 10 V. The relay holding voltage required to hold the switched relay in the switch position, however, is lower than the relay pick-up voltage. A low operating voltage (electrical system voltage) can therefore be high enough to exceed the relay holding voltage and still be lower than the relay pick-up voltage. In this case, it is possible, for example, to hold the relay—which is for switching to the closing action—in the closed switch position, but it is not possible to reverse the actuating drive unit by switching the relay for the opening action since the voltage falls below the required relay pick-up voltage. As a result, the unit to be actuated does not move in the opening direction. A possible result then, as described above, is that the drive unit is switched off after the jamming switch-off time has elapsed.